108. 将有序数组转换为二叉搜索树

https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/description/
思路：递归，根据有序的特性，每次取数组中点mid为根，递归构造左右子树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def sortedArrayToBST(self, nums):
        """
        :type nums: List[int]
        :rtype: TreeNode
        """
        if len(nums) == 0:
            return None
        return self.helper(nums, 0, len(nums)-1)

    def helper(self, nums, low, high):  #辅助函数，对nums[low:high]范围进行树的构造

        if high < low:
            return None
        mid = (low + high) // 2
        root = TreeNode(nums[mid])
        #递归左右子树
        root.left = self.helper(nums, low, mid-1)
        root.right = self.helper(nums, mid+1, high)
        return root
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作者：caisense
来源：CSDN
原文：https://blog.csdn.net/u012033124/article/details/82692220
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